It has been reported for more than 2000 years that Archimedes used mirrors to concentrate sunlight and set Roman ships afire during the seige of Syracuse in 213BC. While much evidence has been presented to refute this claim, it is probably too powerful a legend to die. Nevertheless, the legend supports the saying heard often in the early days of modern solar energy that if solar had been a weapon of war it would have been fully developed by now.
Following the Arab Oil Embargo of 1973-74 and increased U.S. interest in energy issues, the U.S. Department of Energy started a concentrating solar power project called Solar One. It involved hundreds of ground-mounted reflecting mirrors, called heliostats, that followed the sun and directed their sunlight to a water receiver at the top of a 400-foot centrally-located tower.
The heated water was converted into steam and fed into a steam-turbine electricity generator. Construction of Solar One was completed in 1981 and was operational from 1982 to 1986. It was then redesigned to incorporate molten salt (60% sodium nitrate; 40% potassium nitrate) as the thermal collection and storage medium and relabeled Solar Two. The redesign was needed to address the instability of Solar One when sunlight was disrupted by passing clouds. Solar Two was successfully tested at 10MWe using molten salt but operation was eventually discontinued in the mid 90’s when the industry was unwilling to share further development costs with DOE. The Solar Two tower was eventually demolished in 2009; the heliostats are now being used for astronomy research.
CSP also comes in two other ‘flavor’s, parabolic trough and dish-Stirling, both of which are discussed in the attached PowerPoint (‘Concentrating Solar Power’) that I presented in 2010 to a meeting of utility executives. I did so for two reasons, to make sure the executives were familiar with CSP (which had been of limited visibility for a number of years) and to catch up on the current state of the technology which was beginning to reappear.
I will end this blog by emphasizing one of CSP’s major advantages over intermittent renewable energy sources such as PV and wind – it comes with storage. The major barriers to its greater utilization are its requirement for unscattered (direct normal) radiation (you can’t focus scattered sunlight), cost, and the need for cooling (water or air). Deserts, which usually have few clouds and therefore little scattering of sunlight, are natural venues for CSP power plants. Unfortunately, deserts are also known for their lack of water. These issues are discussed in the PowerPoint.